Two-stroke cross-head engines used in marine and/or stationary applications are equipped with two separate lubricating oil systems. One lubricating system comprises so-called system oil that normally is used for lubrication and cooling of the engine's bearings and e.g. oil-cooled pistons as well as for activation and/or control of various valves or the like. The other lubricating system comprises an all-loss lubricant (cylinder oil) that normally is used for lubrication of the engine's cylinders, piston rings and piston skirt.
In typical two-stroke cross-head engines, the cylinder oil is spent continuously by each turn of the engine whereas the system oil in principle is not spent (except by smaller unintentional leakages). The lubrication system comprising the cylinder oil is also often referred to as an “all-loss” lubrication system as the oil is spent. The use of and various types of both system oil(s) and cylinder oil(s) is very well known in the art.
The cylinder oil typically contains certain additives that function to reduce, minimize or neutralise the acid level of the cylinder system.
Typical cylinder oils usually have an SAE (Society of Automotive Engineering) viscosity equivalent to about 50 and normally have a total base number (BN) of about 40 to 70 for the neutralisation of acid products produced during the combustion process. Typical system oils usually have an SAE viscosity of about 30 with a relatively low BN content, typically around 5 and provide long-time performance. These exemplary values may vary dependent on the actual application and the specific design of the systems that the oils are used in.
In recent two-stroke cross-head engine designs involving electronic and/or hydraulic control and/or activation of valves, etc., the minimum performance requirements of the system oil has been substantially increased compared to earlier design using traditional mechanical control/activation.
The performance level of lubricants is typically measured periodically and may not go beyond certain limits if the oiled component's condition should not be jeopardized. One cause of performance loss is caused by particle contamination. These particles include combustion by-products and wear components, which can be partially removed by oil separators. However, in the case of two-stroke cross-head engines, one of the sources of contamination is spent cylinder oil leakage past the stuffing box causing both the viscosity and base number of the system oil to increase over time, a process that cannot be reversed by separators. This causes the system oil to gradually degrade over time so component wear increases and engine efficiency decreases. When the system oil is approaching its condemning limit it will have to be replenished or changed.
Patent specification U.S. Pat. No. 6,074,995 discloses a friction modifier composition comprising an oil of lubricating viscosity and containing a friction-reducing amount of an additive.
Patent specification U.S. Pat. No. 5,282,990 discloses adding a synergistic blend of friction modifiers to a crankcase lubricating oil of an internal combustion engine in order to improve the fuel economy. However, there is no suggestion of how to apply this with respect to a two-stroke diesel engines.
Patent specification EP 0 573 231 discloses triglycerides as friction modifiers added to oil for improved fuel economy in an internal combustion engine.
Patent application US 2003/0171223 discloses lubricating oil compositions with improved friction properties.